The productivity of grade Boran cattle maintained by chemoprophylaxis under severe trypanosomiasis risk was evaluated at Mkwaja Ranch, Tanzania. Twenty thousand calving records collected over a period of 10 years were analysed. It was established in an area of high tsetse challenge that, using a chemoprophylactic drug (Samorin) strategy, acceptable levels of productivity could be obtained. On average, the productivity level achieved at Mkwaja, expressed per unit of metabolic weight of cow maintained, was approximately 80% of that of Boran reared in a tsetse-free ranching environment in Kenya and was 35% greater than trypanotolerant N'Dama kept in tsetse-infested areas of West and central Africa without trypanocidal drugs.